Catalytic reforming is an established refinery process used for improving the octane quality of hydrocarbon feeds and, as noted in U.S. 2004/0129605A1, reforming processes were traditionally operated as semiregenerative or cyclic processes using fixed bed reactors or, more recently, as continuous processes using moving bed reactors. Proposals have also been made for combining fixed and moving bed reactors with the regeneration mode being appropriate to the reactor types used in the hybrid configuration, so that the fixed bed reactors have retained the fixed bed type regeneration, usually semiregenerative, and the moving bed reactors in the unit have retained the dedicated moving bed regenerator. Units of this hybrid type are disclosed, for example, in U.S. 4,498,973; U.S. 5,190,638; U.S. 5,190,639; U.S. 5,5,196,110; U.S. 5,5,211,838; U.S. 5,5,221,463; U.S. 5,354,451; U.S. 5,368,720 and U.S. 5,417,843 as well as in the technical literature, for example, in NPRA Paper No. AM-96-50 “IFP Solutions for Revamping Catalytic Reforming Units” (1996 NPRA Annual Meeting, Mar. 17-19, 1996) which describes a moving bed reforming unit in which two moving bed reactor stacks share a common regenerator. UOP has recently announced its CycleX® Process for increased hydrogen production from a fixed bed reforming unit by the addition of a circulating catalyst reactor as the final reactor in the reactor sequence. This reactor is provided with its own heater and regenerator as an expansion of existing assets rather than as a substitution of them. See NPRA Paper AM-03-93 and the UOP technical data sheet at http://www.uop.com/objects/cyclextechsheet.pdf.
In semiregenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst which is carried out with the catalyst remaining in the reactor. In cyclic reforming, the reactors are individually isolated by various piping arrangements. The catalyst is regenerated and then reactivated while the other reactors of the series remain on line. A “swing reactor” temporarily replaces the reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors with continuous or intermittent addition and withdrawal of catalyst through which the catalyst moves progressively before it is passed to a regeneration zone for regeneration and rejuvenation before being returned once again to the reactor. In the regenerator, at least a portion of the deposited coke is burned off and the regenerated catalyst is recycled to the reactor to continue the cycle. Commercial continuous reforming units may have the reactors arranged in a side-by-side or in a stacked configuration. Because the continuous mode of operation with its frequent regeneration can tolerate a higher rate of coke lay-down on the catalyst, it is possible to operate continuous units at lower and more favorable pressures than those normally used with semi-regenerative and cyclic units in which it is important or at least desirable to extend catalyst life between successive regenerations.
Semiregenerative and cyclic reforming units may be converted to continuous moving-bed units to take advantage of the improved yield of higher octane reformate and hydrogen associated with continuous lower pressure operation but the conversions which have so far been considered are essentially entire unit replacements which require replacement of all existing vessels and most of the ancillary equipment as well as installation of an integrated catalyst regenerator which is one of the most costly items in the conversion. The cost of the regenerator can be as much as about seventy percent of the total cost required for the conversion. U.S. patent applications Ser. Nos. 10/690,081 (Publication No. U.S. 2004/0129605A1) and 60/564,133 describe different conversion techniques by which fixed bed reformers may be converted to moving bed operation without the major expense normally associated with a complete conversion. The technique described in U.S. patent application Ser. No. 10/690,081 replaces the fixed bed reactors with moving bed reactors but retains the existing heaters and piping and eliminates the need for an individual, dedicated regenerator either by utilizing off-site regeneration or by utilizing a regenerator of another unit. The technique described in U.S. patent application Ser. No. 60/564,133, by contrast, enables a fixed bed unit to be converted to a unit with moving bed reactors while converting existing fixed-bed reactor vessels to use as regenerator vessels which are switched in a cyclic sequence between filling, regeneration/rejuvenation and emptying modes.
As noted above, a major advantage of moving bed operation is that it may be operated under a regime in which hydrogen production and conversion of aliphatics to aromatics proceeds to a more favorable equilibrium under the prevailing lower pressure regime. While catalyst coking proceeds at a greater rate under these conditions of reduced hydrogen partial pressure, this is acceptable when the catalyst is regenerated after only a relatively short cycle time in the reactor stack. The conditions normally encountered in continuous, moving bed reforming do, however, impose their own limitations on the choice of catalyst. While fixed bed reformers, whether of the cyclic or semi-regenerative type traditionally used catalysts based on the platinum-rhenium (Pt/Re) combination, these catalysts, even with additional metallic promoters such as iridium, were not optimal for moving bed operation. Under the low pressure conditions used in conventional moving bed reformers, typically below 11 bar (gauge), the normal commercially available catalysts are platinum-tin (Pt/Sn) and this combination of metals is notably more active than the Pt/Re combination for dehydrocyclization at low hydrogen partial pressures. A problem arises, however, with the low cost revamps described in the two applications referred to above, which are intended to provide a route to moving bed reactor operation while avoiding the major expense of regenerator acquisition and replacement of existing ancillary equipment including, especially, the recycle gas compressors and furnaces which are very expensive. The retention of the recycle circuit often limits the magnitude of the pressure reduction which can be achieved during the revamp. If the minimum revamped pressure has to be kept above 11 bar (approx. 160 psig), typically between about 13 and 35 barg (approx. 190 to 510 psig), catalysts based on the Pt/Re combination used at the higher pressures of traditional fixed bed operation may be preferred.
Existing commercial practice in fixed bed reactors using Pt/Re catalysts, enables sufficient coke to be rapidly deposited, even near the inlet of the first reactor, to mitigate the undesirable hydrogenolysis activity associated with the rhenium; in moving bed operation, by contrast, the catalyst closest to the inlet of the first reactor, will consistently have a coke level which is substantially lower than that found in fixed bed operation with Pt/Re catalysts, even shortly after start of run since fresh, uncoked catalyst is continuously added to the inlet of the first reactor. The freshly added Pt/Re catalyst in the first moving bed reactor does not acquire sufficient coke sufficiently quickly to mitigate the inherent hydrogenolysis activity to an acceptable level within a reasonable period of time. Moving bed pilot plant studies with Pt/Re catalyst have shown that even at relatively slow catalyst circulation rates, the steady state coke level anywhere in the lead reactor would be well below 1 weight percent on catalyst. While some of the hydrogenolysis can be mitigated by presulfiding the freshly regenerated Pt/Re catalyst before it is added to the top of the lead reactor, this sulfur is stripped from the catalyst relatively quickly and does not sufficiently suppress hydrogenolysis as the catalyst moves slowly down through the lead reactor. As a result, the refiner seeking to avail himself of a low cost fixed-to-moving bed conversion faces a dilemma: if existing recycle and other equipment is retained, requiring operation at moderate pressure, it may not be economic to use the catalysts which are preferred at moderate pressure conditions because of excessive hydrogenolysis; on the other hand, the Pt/Sn catalysts used in commercial moving bed reformers are not preferred at the higher pressures. Thus, there is a need for resolving this dilemma in a way which permits operation of a revamped unit with the Pt/Re and other catalysts which afford the best performance in the moderate pressure regime imposed by the equipment limitations.